Non-enzymatic browning is a chemical process that produces a brown color in foods without the activity of enzymes. The two main forms of non-enzymatic browning are carmelization and the Maillard reaction. The objectives in cooking meats and other food products, such as hamburgers, etc., is to achieve: (1) good sear on the surface of the food for enhanced flavor, (2) proper internal temperature of the product to ensure against any bacterial contamination, and (3) minimized cooking time to improve speed of service to customer.
Searing is a browning of the meat or other food surface through a process known as the Maillard reaction. When cooking meat, the combination creates the “meaty” flavor and typically changes the color of the food surface. The Maillard reaction occurs most readily at around 300° F. (150° C.) and above. When meat is cooked, the outside reaches a higher temperature than the inside, triggering the Maillard reaction and creating the strongest flavors on the surface. Better sear results in more flavors, which is typically regarded in the industry as a better tasting product.
However, in the case of cooking grills in rapid service restaurants, because water in the form of ice crystals is present at the surface of a frozen meat patty, the initial heat transferred by the cooking platen is used to boil the water off (212° F.), thus inhibiting or delaying the Maillard reaction. This adds time to the cooking to achieve the desired sear. Accordingly, it is desired to minimize the thermal resistance of the cooking surface in contact with the product to allow for desired searing while still attaining needed internal temperature of the foods in the shortest time possible.
The advantage of using PTFE in food preparation devices, such as clamshell-type grills which cook food at high temperatures, is well-known. PTFE has also exhibited utility as a material for use in harsh chemical environments where other polymers quickly degrade. Moreover, PTFE also has a useful operating temperature range from as high as 260° C. to as low as near minus 273° C.
Conventionally, one method to create a non-stick surface on the cooking surface of the grill is to secure a sheet fabricated from conventional non-porous PTFE over the cooking surface. A more common method to create a non-stick surface is to spray the surface, such as a metal or fiberglass substrate, with PTFE, PFA, FEP or other non-stick coatings and bake to solidify. The spray and bake coatings are more susceptible to scratching than a PTFE sheet. While the PTFE sheet prevents sticking of the item being cooked (e.g., hamburgers) to the cooking surface, the sheet is subject to tearing, nicking and scratching, which causes deterioration of the easy release quality of the PTFE. This is attributable to the fact that PTFE is characterized by poor mechanical properties such as low tensile strength, poor cold flow resistance or creep resistance, poor cut-through and abrasion resistance and a general poor mechanical integrity that precludes its consideration in many materials engineering applications. Low porosity PTFE articles have been made in the past through use of a skiving process in which solid PTFE films are split or shaved from a thicker preformed article. These articles are characterized by low strength, poor cold flow resistance, and poor load bearing capabilities in both the length and width dimensions of the film.
A PTFE material, specifically, expanded polytetrafluoroethylene, may be produced as taught in U.S. Pat. No. 3,953,566. Expanded porous polytetrafluoroethylene (“ePTFE”) has a microstructure consisting of nodes interconnected by fibrils. It is of higher strength than unexpanded PTFE but retains the chemical inertness and wide useful temperature range of unexpanded PTFE.
However, ePTFE is porous and hence is less effective as a food preparation surface and cannot be used as a barrier layer to low surface tension fluids since such fluids with surface tensions less than 50 dyne-cm pass through the pores of the membrane.
Compressed ePTFE articles are taught in U.S. Pat. No. 3,953,566 in which a platen press is used to densify a thin sheet of ePTFE with and without heat. However, cold flow occurs in the press and nonuniform parts result and a density of over 2.0 g/cc is difficult to achieve. Also the use of such a platen press greatly limits the scope of width and length of final product which may be produced. Factors including platen parallel surfaces, and ePTFE unit weight variance are also limiting factors with respect to the production of uniformly dense PTFE articles. The ePTFE sheet used in U.S. Pat. No. 3,953,566 was stretched or strengthened in only one direction and hence the utility of the finished article was severely limited.
In U.S. Pat. No. 5,374,473 to Knox et al., a method is described for producing articles of dense ePTFE by placing 2 or more layers of porous ePTFE inside a heat and pressure stable flexible container, evacuating gas from the chamber, subjecting chamber to a pressure of 150 to 350 psi (1034 KPa to 2413 KPa) and temperature from 368 C to 400 C, then cooling the container while reducing pressure. This method of manufacture is mechanically similar to the platen press method cited in U.S. Pat. No. 3,953,566. The scope of length and width which is achievable is clearly limited by the size of the platen press or pressurized container, and the uniformity of the density of the final article is limited by the parallel of the plates used to impart compressive pressure, as well as by the unit weight variance of the porous ePTFE sheet used. These factors serve to further limit the geometrical stature of the final dense ePTFE sheet.
Commercial products currently available from W.L. Gore and Associates, Inc. include a dense fluoropolymer film exhibiting barrier properties. The first product comprises a PTFE barrier layer bonded between two porous PTFE layers. The second product comprises a PTFE barrier layer bonded on one side to a thermoplastic layer such as FEP (fluoroethylene propylene), PFA (perfluoroacrylate) or THV (a polymer of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride). The barrier layer in these commercial products is a film of high water vapor resistance (i.e., low water vapor permeation) PTFE having good tensile properties in orthogonal directions. This barrier layer has a density of 2.0 g/cc or greater, is substantially free of pores, and has a tensile strength of about 8000 psi (55,000 KPa) or greater, more preferably a tensile strength of about 10,000 psi (69,000 KPa) or greater in two orthogonal directions. Mechanical properties such as dimensional stability of these materials have been tailored to meet a range of performance requirements. These materials have been successfully implemented in a number of applications requiring flexible, thin materials with good chemical resistance and water vapor permeation resistance.
Despite the advances in PTFE materials capabilities, a long-felt need has existed for improved surfaces for food preparation devices, such as grill covers, which exhibit a unique combination of easy release, improved thermal resistance, improved non-enzymatic browning, improved tensile properties, and resistance to tearing, nicking and scratching in use.